Composer is a tool for dependency management in PHP. It allows you
to declare the libraries your project depends on and it will manage
(install/update) them for you.

Composer is not a package manager in the traditional sense. It does deal
with "packages" or libraries, but it manages them on a per-project basis,
installing them in a directory (e.g. vendor) inside your project.

Suppose:

a) You have a project that depends on a number of libraries.
b) Some of those libraries depend on other libraries.

Composer:

c) Enables you to declare the libraries you depend on.
d) Finds out which versions of which packages can and need to be
installed, and installs them (meaning it downloads them into your
project).
